Ever been to Sheerness? they reckon it'll do two million pounds worth...

...of improvements.

Seriously though, they more than know and check, it's carefully marked with large conspicuous buoys as well as constantly monitored in case someone fiddles with it and there are some pretty big, fast police RIBs round there. (had a nice friendly chat with them when they wondered if I was a drug smuggler.) Apparently they're cutting off the rusting masts (above water) sometime soon to reduce risk further. But there is not much that sensibly can be done to actually remove it, and there may not be much of it left that can actually go up.

Buried bombs that nobody knows are there are far more worrying... and the Germans have that problem rather worse than Britain does.